A narrow directional microphone is configured so as to house a unidirectional condenser microphone unit in an acoustic tube (interference pipe), and is also called a line microphone, a gun microphone, or the like because the acoustic tube is a slender cylindrical tube having a predetermined axial length.
One example of the configuration of a conventional narrow directional microphone is explained with reference to FIG. 5, which is a longitudinal sectional view thereof, and FIG. 6, which is a sectional view taken along the line B-B of FIG. 5. The narrow directional microphone includes a tube member consisting of an acoustic tube 10 and a microphone casing 20, each consisting of a cylindrical tube, and as a microphone unit, a unidirectional condenser microphone unit 30 is used.
A front end 10a directed to the sound source side of the acoustic tube 10 is open as a front sound wave introduction port 11. A rear end 10b is closed by a gasket 12. In the tube peripheral surface of the acoustic tube 10, side sound wave introduction ports 13 are formed. On the rear end side of the acoustic tube 10, rear sound wave introduction ports 14 are formed.
The side sound wave introduction ports 13 are arranged at two opposed places on the tube peripheral surface with a 180° interval being provided therebetween, and in this example, each of the side sound wave introduction ports 13 is formed as one slit (elongated hole). This slit may be divided into a plurality of slits. Also, in place of the slit, a plurality of round holes are sometimes arranged along the axial direction of the acoustic tube 10.
In any case, as described in Patent Document 1 (Japanese Patent Application Publication No. 2000-50385), to the side sound wave introduction port 13, an acoustic resistance material 15 consisting of nonwoven fabric, nylon mesh; or the like is affixed. To the rear sound wave introduction port 14 as well, an acoustic resistance material of the same kind is sometimes affixed.
The microphone casing 20, which is larger in diameter and longer than the acoustic tube 10, is put coaxially over the acoustic tube 10. In the tube peripheral surface of the microphone casing 20, openings 21 are formed in portions facing the side sound wave introduction ports 13 and the rear sound wave introduction ports 14.
Between the microphone casing 20 and the acoustic tube 10, a wire mesh 22 for shielding extraneous electromagnetic waves is provided. In the tube of the microphone casing 20 extending to the rear (the right-hand side in FIG. 5) beyond the acoustic tube 10, a circuit board having a sound signal output circuit and the like is housed.
The condenser microphone unit 30 is disposed between the side sound wave introduction ports 13 and the rear sound wave introduction ports 14 in the state in which in the acoustic tube 10, a front acoustic terminal 30a is directed to the front sound wave introduction port 11 side, and a rear acoustic terminal 30b is directed to the rear sound wave introduction port 14 side. Signal output wires 31 of the condenser microphone unit 30 are pulled out via an insertion hole formed in the gasket 12, and are connected to the predetermined terminals of the circuit board.
In the narrow directional microphone, the acoustic resistance material 15 provided especially on the side sound wave introduction port 13 exerts an important influence on the sensitivity, frequency response, and directivity. Conventionally, an adhesive is applied manually to the acoustic resistance material 15 to affix it. Therefore, the acoustic resistance is varied by the oozing of adhesive, the way of affixing, and the like, which poses a problem in terms of quality control.
Also, concerning the condenser microphone unit 30 used, the larger the diameter thereof, the larger the effective vibration area. Therefore, the condenser microphone unit 30 having a large diameter is advantageous in terms of high sensitivity and low noise. However, the diameter of the unit 30 is restricted by the inside diameter of the acoustic tube 10.
In the above-described conventional example, since the wire mesh 22 for shielding extraneous electromagnetic waves must be provided between the microphone casing 20 and the acoustic tube 10, the diameter of the acoustic tube 10 decreases accordingly. Therefore, there also arises a problem that it is difficult to mount a large-diameter condenser microphone unit providing high sensitivity and low noise.
To solve this problem, the diameters of both the microphone casing 20 and the acoustic tube 10 have only to be increased. In this case, however, the whole of microphone becomes large in size and heavy in weight. Therefore, the compactness and lightweight required for the narrow directional microphone are sacrificed, so that this solution cannot be said to be preferable.
Accordingly, an object of the present invention is to provide a narrow directional microphone in which the acoustic resistance of an acoustic tube scarcely varies, and a larger-diameter condenser microphone unit can be used without increasing the external dimensions.